Production of gaseous hydrogen fluoride

ABSTRACT

A process for preparing substantially pure gaseous hydrogen fluoride from ammonium bifluoride by reacting a product containing ammonium bifluoride with a metal fluoride or a double fluoride of ammonium and metal at a temperature up to 500* C. to produce substantially pure gaseous hydrogen fluoride and a fluometallate. The fluometallate can thereafter be decomposed into gaseous ammonium fluoride and the metal fluoride or double fluoride of ammonium and metal. The process is conducted under anhydrous medium reaction conditions, and the starting materials are solid prior to being heated.

United States Patent Bachelard 1 Feb. 29, 1972 [54] PRODUCTION OFGASEOUS References Cited HYDROGEN FLUORIDE UNITED STATES PATENTS [7212,659,655 11/1953 Sweet ..23/88 x [73] Assignee: Ugine Ruhlmann, Paris,France 3,005,685 10/1961 Riedlet al..... ....23/88 3,195,979 7/1965Burkert et al ..23/153 1 Flled= July 8,197" 3,338,673 8/1967 Peterson6161. .....23/153 3,476,509 11/1969 Jones ..23/50 [2]] Appl' 533003,525,584 8/1970 K1dde ..23/88 Related US. Application Data PrimaryExaminerEdward Stern [63] Continuation-in-part of Ser. No. 737,327, June17, Atwmey webb, Burden, Robinson and Webb 1968, abandoned.

[57] ABSTRACT [30] Foreign Application Priority Data A process forpreparing substantially pure gaseous hydrogen May 30,1967 France..108352 fluoride from ammonium bifluoride by reacting a product Oct.27, 1967 France ..126140 containing ammonium bifluoride with a metalfluoride or a double fluoride of ammonium and metal at a temperature up[52] U.S.Cl ..23/153, 23/19, 23/21, to 500 C. to produce substantiallypure gaseous hydrogen 23/88, 23/193, 23/345, 23/352 fluoride and afluometallate. The fluometallate can thereafter [51] lnt.Cl ..C0lb7/22,C0lb9/08,C0lb l/l6 be decomposed into gaseous ammonium fluoride andthe [58] Field of Search ..23/153,50, 88, 193,19 V, metal fluoride ordouble fl oride of mm ni m an m l- 23/345, 352 The process is conductedunder anhydrous medium reaction conditions, and the starting materialsare solid prior to being heated.

11 Claims, No Drawings PRODUCTION OF GASEOUS HYDROGEN FLUORIDE This is acontinuation-in-part application of Ser. No. 737,327, filed June 17,1968, now abandoned, having the same title.

Gaseous mixtures are discharged in many industrial processes which, asresidues or impurities, contain fluoro compounds. Frequently, thesecompounds are in the form of hydrofluoric acid or silicon tetrafluoride,or their mixtures.

This invention relates to the production of substantially pure gaseoushydrogen fluoride and, more particularly, to an economic process forproducing this hydrogen fluoride from ammonium bifluoride recovered fromindustrial waste gases.

Different methods have been suggested for the elimination of thefluoroderivatives from these gaseous mixtures. Most of these methods arebased upon washing or scrubbing the gases. Some of the methods result ina more or less concentrated solution of fluosilicic acid. Others employammonia and result in solutions of ammonium fluoride after theseparation of silica by filtration. The ammonium fluoride solutions,regardless of how they are produced, may be heated at a temperaturenonnally lying between 60 and 200 C., to drive off the ammonia, toobtain anhydrous ammonium bifluoride or a practically anhydrous mixturevery rich in bifluoride.

Attempts have been made to extract the fluorine contained in thefluorides in the form of hydrofluoric acid. For example, it has beensuggested that anhydrous ammonium bifluoride be reacted withconcentrated sulphuric acid at approximately 190 C., whereby up to 98percent of the fluorine present in the bifluoride may be recovered inthe form of anhydrous hydrofluoric acid. The ammonia is recovered asammonium bisulphate. The recovery of ammonia as a low-value salt is aserious disadvantage to the production of hydrofluoric acid by thisprocess. Moreover, the process must be carried out in a very specialplant because of the corrosive environment. In fact, it is verydifiicult because of the unavoidable and wellknown corrosion problem.These drawbacks render it impossible to apply an industrial cycle inwhich bases containing fluoro products, absorbable by an ammoniacalsolution, could be washed with this solution, heated to obtain ammoniumbifluoride, and the bifluoride processed with concentrated sulphuricacid to obtain the anhydrous hydrofluoric acid.

I have invented a novel process for the treatment of ammonium bifluoridein order to extract substantially pure gaseous hydrogen fluoridetherefrom without the disadvantages of the prior methods. This novelprocess does not utilize sulphuric acid and the problems caused bycorrosion are negligible. My invention is based on the property ofnumerous metals, such as Bi, Cd, Co, Cu, Mn, Zn, Mg, Fe, Ni, Ge, Sn, Th,Ti, Al, ln, Zr, Cr, Be, Ga, V, Sc, Pb, Hf, Sb, and U, in particular, ofyielding stable ammonium fluometallates or double fluorides of metal andammonium which are decomposable by heat, and render it possible toeffect separate recovery of the metal fluoride and ammonium fluoride forsubsequent use. It is known that the fluometallates formed are stableand, further, they are decomposable by heat.

Briefly stated, my invention consists of reacting solid ammoniumbifluoride or a product containing ammonium bifluoride with a solidmetal fluoride to form a fluometallate and release substantially puregaseous hydrogen fluoride. The temperature of the reaction should be atleast equal to that of the point of fusion of the reagent with thelowest fusion point but less than the temperature of decompositionoffluometallate formed. After the release of gaseous hydrogen fluoridehas substantially ceased and it has been collected, the temperature isincreased to a temperature which will decompose the fluometallate.Ammonium fluoride is collected in the gaseous stage and metal fluorideis collected in the condensed stage. The metal fluoride may then berecycled.

In the process I prefer to use a simple metal fluoride chosen, forexample, from the fluorides of Bi, Cd, Co, Cu, Mn, Zn, Mg, Fe, Ni, Ge,Sn, Th, Ti, Al ln, Zr, Cr, V, Be, Ga, Sc, Pb, Hf, Sb and U. On the basisof tests, experimentation and analysis, it has been determined that theabove-identified metal fluorides will react with a solid ammoniumbifluoride compound to form substantially pure gaseous hydrogen fluorideand an ammonium fluometallate under the subject process conditions. Adouble fluoride of ammonium and metal may also be used. lf a doublefluoride is used, it must have a lower index than the double fluorideformed. Thus, double fluorides of ammonium and of metals such as Al, Fe,Zr, In, Cr, U and V are satisfactory.

Using a trivalent metal as an example, the reactions of the processusing a simple metal fluoride are:

4)a a a+ 4 (8 (2) The reactions in the process using a double fluoridemay be written as follows:

(Nl-l MF., NH MF +2NH,F (gas) 4 It is apparent that the process rendersit possible to recover the quantity of fluorine present in thebifluoride employed, in the form of ammonium fluoride.

The initial heating of the mixture of solid ammonium bifluoride andsolid metal fluoride may be continued to complete the conversion intofluometallate and gaseous hydrogen fluoride. A rate of conversion of I00per cent cannot, however, be achieved since, in addition to thefluometallate required, the resulting solid contains unconverted metalfluoride and ammonium bifluoride. However, since this solid is exposedto a second heating action, the metal fluoride and the ammoniumbifluoride will be recovered either as they are or in the form of theirdecomposition products, with the solid and gaseous products emergingfrom the second heating action.

The quantity of metal fluoride employed is at least equal to thequantity corresponding stoichiometrically to the bifluoride empolyed.The temperature applied to cause reaction between the ammoniumbifluoride and the metal fluoride depends on the metal employed.Although temperatures of up to 500 C. may be utilized, temperatureslying between and 200 C. are preferable. It is preferable to applyrelatively low temperatures, in particular, temperatures only slightlyabove the melting temperature of the bifluoride employed in order toprevent pollution of the gaseous hydrogen fluoride by ammonium fluoride.The formation by decomposition of certain fluometallates is appreciableat temperatures such as 200 C.

It may be advantageous to operate under a pressure slightly greater thanatmospheric pressure in order to slow the rate of decomposition of thefluometallate formed. The higher pressure permits use of a relativelyhigh temperature in promoting the kinetics of gaseous HF release withoutencountering the drawbacks of inopportune release of ammonium fluoride.

The temperature used for the decomposition of the fluometallate isclosely linked with the metal employed. The fluometallates that may beemployed according to the invention decompose at speeds which dependupon the temperature applied, and a heating temperature will be chosenfor each of these for which its speed of decomposition is optimum. Ingeneral, the temperature chosen will be distinctly higher than thetemperature used for the formation of the fluometallate with release ofthe hydrogen fluoride required.

It is clear that the process described may be applied successfully in acycle for the recovery of fluoro products from a gaseous mixturecontaining these. A cycle of this nature comprises washing the gaseswith an ammoniacal solution according to known processes, heating thesesolutions to extract solid ammonium bifluoride and a quantity of gaseousammonia corresponding to that which has been absorbed by the fluoroproducts during washing, and then treating the bifluoride according tothe process herein described. It is plain that a cycle of this kindrecovers the absorbable fluoro products contained initially in thegaseous mixture, in the form of HF, without this extraction requiring aconsumption of ammonia or of some other reactant.

The following examples are illustrative of my invention:

EXAMPLE 1 A solution of ammonium fluoride was obtained by washing gascontaining hydrofluoric acid and silicon tetrafluoride coming from aplant producing superphosphates.

The gaseous effluent from the decomposition of ammoniumhexafluoaluminate resulting from a prior operation, was added to thissolution. This solution was heated to 180 C. to obtain molten ammoniumbifluoride and gaseous ammonia. The solid bifluoride was mixed withanhydrous aluminum fluoride in excess relative to the stoichiometricalamount. This mixture was placed in a closed reactor, equipped with a gasinlet and outlet, and placed in a progressively heated oil bath. Whenthe reactor reached 147 C., gaseous HF was released at a rate thatincreased rapidly with the temperature. The entrainment of HF wasassured by means of a flow of nitrogen and a slight overpressuremaintained in the reaction enclosure (9 cms. of mercury aboveatmospheric pressure). At the end of 1% hours at these temperatures andpressure conditions, more than 90 per cent of the hydrofluoric acid wasreleased and recovered. No ammonia could be detected in the acidobtained.

The residual solid consisting principally of ammonium fluoaluminate(also known as ammonic cryolite), aluminum fluoride and nondecomposedammonium bifluoride, was heated to 450 C. The gaseous mixture released,principally consisting of ammonium fluoride combined with the gaseousdecomposition products of ammonium bifluoride, was recycled into thesolution of ammonium fluoride. The solid resulting from the same heatingaction was mixed with a fresh batch of bifluoride. The gaseous ammoniareleased during conversion of the solution of ammonium fluoride intobifluoride was employed again for treating another volume of gas, thusforming another batch of solution of ammonium fluoride.

Substantially pure gaseous hydrogen fluoride was obtained, practicallywithout consumption of reactant.

EXAMPLE 2 As intimate a mixture as possible of g. of ammonium bifluorideoriginating from the evaporation, according to known processes, of asolution of ammonium fluoride and 10 g. of anhydrous aluminum fluoride(A1F resulting from the thermic decomposition of ammonic cryolite formedduring an earlier cycle in a Monel metal reactor without heating. Thequantity of AlF exceeded the stoichiometrical quantity by 32 per cent. 7a

When the mixture was homogeneous, the reactor was closed and subjectedto progressive heating in an oil bath. Scavenging with nitrogenestablished an overpressure of 5 cms. of mercury in the reactionenclosure. At 180 C., the release of gaseous hydrogen fluoride becamenoticeable. The process evolved at constant temperature with increasingspeed which reached its maximum value at the end of 32 minutes when therate of reaction amounted to 45 per cent. The temperature was maintainedthroughout the test, which was completed after 2 hours with an HF yieldof 97.1 per cent, and the yield exceeded 95 per cent after 75 minutes.

Dosing disclosed the total absence of ammonia from the hydrogen fluoriderecovered.

The residue of the operation appeared in the form of a relatively hardsintered cake. Heating the cake to 450 C.,- allowed recovery of thequantity of A11 employed, on theone hand, and of a gaseous ammoniumfluoride corresponding in quantity to the bifluoride decomposed, on theother hand. These two products could be employed again in another cycleof operation.

EXAMPLE 3 19.327 g. of ZrF was mixed intimately with 9.899 g. ofammonium bifluoride, in such manner as to have an excess of 100 per centof ZrF,. These reactants were placed in the device described in thepreceding example. The flow of nitrogen assuring entrainment of HFestablished a relative overpressure of 5.5 cms. of mercury in thereactor. A release of gaseous hydrogen fluoride made its appearance from98 C. onwards, and the speed of reaction rose quickly, since thetemperature increase was continued.

The process evolved according to the reaction equation At the end of 10minutes, at 123 C., the HF yield was 38.5 per cent. It reached 71.0 percent at 136 C. at the end of 20 minutes, and 92 per cent at C. after 35minutes. The test was finally interrupted at C., reached atthe end of 92minutes, and the final yield of the reaction amounted to 97.4 per cent.

No ammonia was detected in the product collected.

The residue had the appearance of a porous crumbly mass. Evaluation ofthe rate of reaction by weight loss showed a yield of 98.9 per centwhich is valid in view of the difficulty of recovering the solidtotally.

19.280 g. of ZrF ready for reuse, was recovered by heating the residualsolid to 450 C.

EXAMPLE 4 In the following example, the double fluoride of aluminum andammonium are utilized according to the reaction:

(NH AlF NH AIF +2NH F at temperature s 300 C.

9.152 g. of ammonium bifluoride was intimately mixed with 1 1.011 g. ofammonium tetrafluoaluminate corresponding to the accurate analysis (AlF'1.057 NH F). This ground mixture was poured into a reactor subjected toheating in an oil bath equipped with a thermostat. The temperature ofthe oil was regulated at C. The gaseous hydrogen fluoride released wasrecovered in water at the outlet of the reactor. The test lasted 2 hoursand 10 minutes. The reaction yield was 92 per cent in HF. The solidresidue, 17.208 g., contained 15.67 g. of ammonic cryolite. This solidresidue was placed into an iron reactor subjected to electric heating.After a 45 minute heating ended at 275 C., there was noted a loss inweight of 8.02 g. corresponding to the release of NH and HF. There was9.19 g. of a solid corresponding to the formula AIR-1.07 NH F. Thisproduct could be used again in the first reaction. The gases releasedwere recovered by condensation in the form of ammonium fluoride whichcould also be used again.

The reactions set in action in the process, when utilizing a doublefluoride of zirconium, are:

EXAMPLE 5 0.24 mols of ammonium bifluoride was added to one-tenth mol ofyellow lead oxide PbO. When the reagents were intimately mixed they wereplaced in a Monel metal reactor equipped with a scavenging device by aninert gas and with a steam condensation system. I

in a first step the mixture was heated to 160 C. for 2 hours, thenprogressively raised to 300 C. and maintained at this temperature for 4hours.

The Pb and N11 complex fluoride thus formed was dissociated. The solidresidue consisted of about 0.1 mol of lead fluoride PbF This compoundwas then carefully mixedwith 0.2 mols of NILHF and the whole, still inthe Monel metal reactor, was heated to 160 C. under dry air scavenging.The reaction occurred as follows:

PbF +NH F-HF PbF .2NH F+2HF The gaseous hydrogen fluoride released andwas collected in water.

At the end of 4 hours under these conditions, 93 per cent of theyhydrofluoric acid stoichiornetrical quantity was recovered. Then thedotible fluoride of lead and ammonium was thermally decompos d in dr'y'atmosphere, which allowed to recycle PbF EXAMPLE 6 EXAMPLE 7 andcorresponds to the general formula NH F, 0.2 HF. The.

washing solution contains 16.6 g. of NH;,. The reactions are:

This solid is used to prepared a solution saturated at 90 C. Anevaporation of this liquid phase is then performed to obtain, accordingto known processes, at 200 C., ammonium difluoride. The operationproduces, in fact, 147 g. of a product whose formula is NH.,F,0.95HF; 28g. of NH are liberated and recovered by washing. The reaction is thefollowing:

;.NH,F,0.2 HF 200 C. NH,F, 0.95 HF+0.628 NH,

Besides we have evaporated 01635 liter orfiamfiafif NH F at 300 g./lconcentration, resulting from the treatment of 456 g. of a solution of HSiF at 27 percent with the whole of ammoniacal solutions formed duringthe operation and filtration of the silica formed. At 200 C. 147 g. ofNH F, 0.95 HF have been obtained. The reactions are:

The whole of difluoride (394 g.) is crystallized, crushed and finallymixed with the residue originating from the first operation. A heatingto 150 C. under 0.15 atm. in a Monel reactor causes a release of 100 g.of HF after 40 minutes. The gas is compressed under atmospheric pressureand then washed, first with an azeotropic solution containing 38.25percent of HF maintained at 15 C., then with water. The reaction is:NH.,FeF +2Nl-l,F,0.95 HF (NH FeF +l.90HF The yield is 95 percent in HF.

EXAMPLE 8 A sample of antimony oxide Sb O weighing 33 g. was placed in avessel whose walls were covered with PTFE (teflon). To this sample wasadded a solution containing 40 percent of hydrofluoric acid in excessrelative to the stoichiometrical reaction which involves formation ofSbF The whole was subjected to a slow evaporation in water-bath at 90 C.At the end of several hours the solution began to deposit crystals. Theevaporation was continued till obtaining a slurry. The suspension wasthen cooled down and the solid was filtered, washed with alcohol anddried. Finally 29 g. of SbF; were collected. The antimony fluoride wasremoved to a Monel metal reactor in which it was mixed with l 1.5 g. ofammonium difluoride resulting from dry evaporation of an ammoniumfluoride solution at 300 g./l. This ammonium fluoride liquor was itselfformed by neutralizing with ammonia at pH 9 a solution containing 18percent of fluosilicic acid. The Monel reactor containing the solids SbFand NH F, HF was placed in an oil-bath equipped with a thermostat andthe temperature was raised to l30 C. From l25 C. there was an importantrelease of hydrofluoric acid which was entrained by an inert gaseouscurrent and absorbed by water. At the end of 1% 1 gpsmgmbas HF)+2.92 NHai hours, 3.7 g. of hydrofluoric acid were collected, i.e., that theyield reached 92 percent compared to the difluoride utilized. The doublefluoride of Sb and NH was then decomposed under vacuum at 200 C. and theproducts collected were recycled.

EXAMPLE 9 First 0.1 mol of thorium nitrate was treated by dry processwith a reagent mixture containing 0.48 mol of ammonium difluoride and0.36 mol of ammonium fluoride. The reagents placed in a Monel metalreactor were progressively heated to 280 C. under scavenging withnitrogen till cessation of the gaseous release. The residue was washedwith methanol then dried. About 30 g. of a white powder which, subjectedX-rays, was admitted to be thorium fluoride were collected. In a secondstep the thorium fluoride thus prepared was mixed with 0.2 mol ofammonium difluoride. The whole was removed to a Monel metal closedreactor outwardly heated in an oil-bath. The reagents were raised to 150C. as quickly as possible. From 126 C. an important release ofhydrofluoric acid was noted. The acid was entrained by air and washedwith water. At the end of 1 hour 3.8 g. of hydrofluoric acid werecollected, i.e., that the yield reached 95 percent. All of the ammoniacontained in difluoride was fixed on thorium fluoride in the form of acomplex. The latter was then thermally decomposed about 280 C. and,within 3 hours, the whole of the primary thorium fluoride was recovered.The ammonium fluoride released during this operation was collected in asolid state by condensation. It was used for increasing theconcentration of a NH F exH SiF solution which, by evaporation andthermal decomposition, will produce the ammonium difluoride required forthe cycle.

lclaim:

l. A process for the preparation of pure gaseous hydrogen fluoride fromsolid ammonium bifluoride and products containing ammonium bifluoridecomprising, reacting a solid ammonium bifluoride compound with an amountof a solid metal fluoride at least equal to the quality correspondingstoichiometrically to said bifluoride, said solid metal fluoride beingcapable of yielding a stable ammonium fluometallate when reacted withammonium bifluoride at a temperature above the fusion temperature of atleast one of said ammonium bifluoride and the metal fluoride to obtainthe quantitative release of hydrogen fluoride contained in saidbifluoride and form substantially pure gaseous hydrogen fluoride and anammonium fluometallate while maintaining the temperature between thefusion temperature of the reactant with the lowest fusion temperatureand below the temperature at which the ammonium fluometallate decomposesand recovering the gaseous hydrogen fluoride, said reaction beingcarried out in an anhydrous medium.

2. The process as set forth in claim 1 wherein the metal fluoride isselected from the group consisting of the fluorides of Bi, Cd, Be, Co,Cu, Fe, Mg, Mn, Ni, Sn, Th, Ti, Zn, Al, in, Zr, Cr, V, Ge, U, Ga, Sc,Pb, Hf and Sb. 1 3. The process as set forth in claim 1 wherein themetal fluoride is a double fluoride of ammonium and a metal selectedfrom the group consisting of Al, Fe, Zr, In, Cr, V and U.

4. The process as set forth in claim 1 wherein the temperature ismaintained between and 200 C.

5. The process as set forth in claim 1 wherein the reaction is carriedout at a pressure above atmospheric pressure.

6. A cyclic process for the preparation of pure gaseous hydrogenfluoride from solid ammonium bifluoride and products containing ammoniumbifluoride comprising the steps of:

A. Reacting the solid ammonium bifluoride compound with an amount of asolid metal fluoride at least equal to the quantity correspondingstoichiometrically to said bifluoride, said solid metal fluoride beingcapable of yielding a stable ammonium fluometallate when reacted withammonium bifluoride at a temperature above the fusion temperature of atleast one of the ammonium bifluoride and the metal fluoride to obtainthe quantitative release of hydrogen fluoride contained in saidbifluoride and form substantially pure gaseous hydrogen fluoride and anammonium fluometallate while maintaining the temperature between thefusion temperature of the reactant with the lowest fusion temperatureand below the temperature at which the ammonium fluometallatedecomposes;

B. Maintaining said temperature until the evolution of pure gaseoushydrogen fluoride substantially ceases;

C. Recovering the gaseous hydrogen fluoride; and,

D. Increasing the temperature to a temperature above the decompositiontemperature of the ammonium fluometallate to decompose it into ammoniumfluoride and the metal fluoride used in step A.

7. The process as set'forth in claim 6 wherein the metal fluoride isselected from a group of metals consisting of Bi, Cd, Be, Co, Cu, Fe,Mg, Mn, Ni, Sn, Th, Ti, Zn, Al, In, Zr, Cr, V, Ge, U, Ga, Sc, Pb, Hf andSb.

8. The process as set forth in claim 6 wherein the metal fluoride is adouble fluoride of ammonium and a metal selected from the groupconsisting of Al, Fe, Zr, in, Cr, U and V.

9. The process as set forth in claim 6 wherein temperature of step A isbetween about 125 and 200 C.

10. The process as set forth in claim 6 wherein the pressure of thereaction is slightly above atmospheric.

11. The process for the production of pure gaseous hydrogen fluoridefrom industrial waste gases containing fluoroderivatives absorbable byammonia comprising:

A. Washing the industrial waste gases with ammonia to produce a solutioncontaining ammonium fluoride;

B. Heating the solution containing ammonium fluoride v liberatinggaseous ammonia and extracting a product containing ammonium bifluoride;

C. Reacting the product containing ammonium bifluoride with an amount ofa solid metal fluoride at least equal to the quantity correspondingstoichiometrically to said bifluoride, said solid metal fluoride beingcapable of yielding a stable ammonium fluometallate when reacted withammonium bifluoride at a temperature above the fusion temperature of atleast one of the ammonium bifluoride and the metal fluoride to obtainthe quantitative release of hydrogen fluoride contained in saidbifluoride and form gaseous hydrogen fluoride and ammonium fluometallateat a temperature between the fusion temperature of the reactant with thelowest fusion temperature and below the temperature at which theammonium fluometallate decomposes and maintaining said temperature untilthe evolution of gaseous hydrogen fluoride substantially ceases;

D. Recovering the gaseous hydrogen fluoride;

E. increasing the temperature to a temperature above the decompositiontemperature of the ammonium fluometallate to decompose it into ammoniumfluoride and the metal fluoride used in step C;

F. Collecting the gaseous ammonium fluoride liberated in step E andrecycling it for use in step B; and

G. Collecting the metal fluoride from step E and recycling it for use instep C.

UNITED STATES PA ENT ()Fl lcl'a CERTIFICATE OF CORRECTION P t n N 3x645,681 Dated February 29,1972

I fl-i Roland Bachelard It is certified that error appears inftheabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1 Line 38 -bases-- should read --gases--. Column 4 Line 68 -NHshould read --2NI-I Claim 1 column 6 Line 40 -qua.lity should readque1ntity-.

Signed and sealed this 11th day of July 1972.

Commis sioner' of Patents Attesting Officer FORM po-uoso (1069)

2. The process as set forth in claim 1 wherein the metal fluoride isselected from the group consisting of the fluorides of Bi, Cd, Be, Co,Cu, Fe, Mg, Mn, Ni, Sn, Th, Ti, Zn, Al, In, Zr, Cr, V, Ge, U, Ga, Sc,Pb, Hf and Sb.
 3. The process as set forth in claim 1 wherein the metalfluoride is a double fluoride of ammonium and a metal selected from thegroup consisting of Al, Fe, Zr, In, Cr, V and U.
 4. The process as setforth in claim 1 wherein the temperature is maintained between 125* and200* C.
 5. The process as set forth in claim 1 wherein the reaction iscarried out at a pressure above atmospheric pressure.
 6. A cyclicprocess for the preparation of pure gaseous hydrogen fluoride from solidammonium bifluoride and products containing ammonium bifluoridecomprising the steps of: A. Reacting the solid ammonium bifluoridecompound with an amount of a solid metal fluoride at least equal to thequantity corresponding stoichiometrically to said bifluoride, said solidmetal fluoride being capable of yielding a stable ammonium fluometallatewhen reacted with ammonium bifluoride at a temperature above the fusiontemperature of at least one of the ammonium bifluoride and the metalfluoride to obtain the quantitative release of hydrogen fluoridecontained in said bifluoride and form substantially pure gaseoushydrogen fluoride and an ammonium fluometallate while maintaining thetemperature between the fusion temperature of the reactant with thelowest fusion temperature and below the temperature at which theammonium fluometallate decomposes; B. Maintaining said temperature untilthe evolution of pure gaseous hydrogen fluoride substantially ceases; C.Recovering the gaseous hydrogen fluoride; and, D. Increasing thetemperature to a temperature above the decomposition temperature of theammonium fluometallate to decompose it into ammonium fluoride and themetal fluoride used in step A.
 7. The process as set forth in claim 6wherein the metal fluoride is selected from a group of metals consistingof Bi, Cd, Be, Co, Cu, Fe, Mg, Mn, Ni, Sn, Th, Ti, Zn, Al, In, Zr, Cr,V, Ge, U, Ga, Sc, Pb, Hf and Sb.
 8. The process as set forth in claim 6wherein the metal fluoride is a double fluoride of Ammonium and a metalselected from the group consisting of Al, Fe, Zr, In, Cr, U and V. 9.The process as set forth in claim 6 wherein temperature of step A isbetween about 125* and 200* C.
 10. The process as set forth in claim 6wherein the pressure of the reaction is slightly above atmospheric. 11.The process for the production of pure gaseous hydrogen fluoride fromindustrial waste gases containing fluoroderivatives absorbable byammonia comprising: A. Washing the industrial waste gases with ammoniato produce a solution containing ammonium fluoride; B. Heating thesolution containing ammonium fluoride liberating gaseous ammonia andextracting a product containing ammonium bifluoride; C. Reacting theproduct containing ammonium bifluoride with an amount of a solid metalfluoride at least equal to the quantity corresponding stoichiometricallyto said bifluoride, said solid metal fluoride being capable of yieldinga stable ammonium fluometallate when reacted with ammonium bifluoride ata temperature above the fusion temperature of at least one of theammonium bifluoride and the metal fluoride to obtain the quantitativerelease of hydrogen fluoride contained in said bifluoride and formgaseous hydrogen fluoride and ammonium fluometallate at a temperaturebetween the fusion temperature of the reactant with the lowest fusiontemperature and below the temperature at which the ammoniumfluometallate decomposes and maintaining said temperature until theevolution of gaseous hydrogen fluoride substantially ceases; D.Recovering the gaseous hydrogen fluoride; E. Increasing the temperatureto a temperature above the decomposition temperature of the ammoniumfluometallate to decompose it into ammonium fluoride and the metalfluoride used in step C; F. Collecting the gaseous ammonium fluorideliberated in step E and recycling it for use in step B; and G.Collecting the metal fluoride from step E and recycling it for use instep C.